1. Field of the Invention
The present invention relates to power converters (e.g., boost converters). In particular, the present invention relates to isolated power converters, such as boost converters.
2. Discussion of the Related Art
Boost converter topologies have been extensively used in various AC/DC and DC/DC power conversion applications. In fact, virtually all the front ends of today's AC/DC power supplies that include a power-factor correction (PFC) feature are implemented using a boost converter topology. Boost converter topologies are also used in numerous applications where a relatively low battery-powered input voltage is used to generate a high output voltage. The vast majority of conventional power converters that use a boost topology are non-isolated. However, in some applications, boost converters with galvanically-isolated input and output are required.
Isolated boost converter having one or more isolation transformers are known and studied. Typically, an isolated boost converter may have one or more switches and one or more boost inductors. For example, FIG. 1 shows a current-fed push-pull converter 100, which was disclosed in U.S. Pat. No. 3,938,024 to P. W. Clarke, entitled “Converter Regulation by Controlled Conduction Overlap,” and issued on Feb. 10, 1976. As shown in FIG. 1, push-pull converter 100 includes an isolated boost converter with boost inductor 101 and switches 102a and 102b. As another example, FIG. 2 shows isolated boost converter 200, which was disclosed in the article “A Current-Sourced Dc—Dc Converter Derived via the Duality Principle from the Half-Bridge Converter” by P. J. Wolfs, IEEE Trans. Industrial Electronics, vol. 40, pp. 139-144, February 1993, uses boost inductors 101a and 101b, and switches 102a and 102b. 
Isolated boost converter 200 has a simpler transformer design than single-inductor boost converter 100, in that primary winding 204 and secondary winding 205 in transformer 203 each require only a single winding. By contrast, single-inductor boost converter 100 requires tapped primary and secondary windings 104 and 105 (i.e., essentially two windings each in the primary and secondary windings). In addition, the voltage stress on each of switches 102a and 102b in isolated boost converter 200 is one-half the voltage stress on each of switches 102a and 102b of single-inductor isolated boost converter 100. In particular, the voltage stress on each of primary switches 102a and 102b of isolated boost converter 200 equals the reflected output voltage to primary winding 204, whereas the voltage stress on each primary switch 102a and 102b in isolated boost converter 100 equals twice the reflected output voltage. In either isolated boost converters, however, an increased voltage stress on the primary switches results from ringing between the parasitic leakage inductance of the transformer and the output capacitance of the primary switch (i.e., primary switch 102a or 102b) that is being turned off.
FIG. 3 shows isolated boost converter 300, which does not suffer from the increased voltage stress on the primary switches discussed above. Isolated boost converter 300 was disclosed in the article “New Single-stage PFC Regulator Using Sheppard-Taylor Topology,” by C. K. Tse and M. H. L. Chow, IEEE Trans. Power Electronics, vol. 13, pp. 842-851, September 1998. In isolated boost converter 300, the maximum voltage stress on each of primary switches 102a and 102b—which are turned on and off simultaneously—is clamped to the voltage of energy-storage capacitor 306 by clamp rectifiers 307a and 307b. However, isolated converter 300 suffers from severe parasitic ringing across the primary winding 204 of transformer 203 due to the parasitic resonance in the leakage inductance of transformer 203 and the junction capacitance of rectifier 308. The parasitic resonance degrades the performance isolation boost converter 300.